The present invention is directed generally to signal sensing devices for sensing signal parameters such as current and power. More particularly, the present invention relates to the construction and use of such devices for sensing signals in radio frequency ("rf") systems which are fabricated on substrates using integrated circuit fabrication techniques. Still more particularly, the present invention concerns a signal sensor structure which is fabricated as part of an rf integrated system. In a further aspect, the present invention involves an rf power sensor for sensing power in rf circuits without ohmic losses or high inductive impedances, while providing a sense signal that is only 10-20 dB smaller than the rf signal to be sensed with negligible phase change.
Monolithic rf devices such as transceivers, amplifiers and the like formed on substrates such as silicon are of interest because they have the potential to be fabricated at low cost in large volumes, and can be designed for low power consumption, i.e. portable applications. Power detection is essential in those systems in order to adjust the power to the minimum required levels. Power sensing as part of feedback loops can help to improve the linearity of building blocks because biasing conditions can be adjusted to eliminate nonlinear signal components. The number of required devices, such as filters and the like, can thus be reduced. As a result of the reduced device count, noise is minimized such that lower power levels may be used.
The simplest known power sensor, a series resistor, is lossy and provides a sense signal with the same potential as that of the power port. A potential-independent sense signal can be provided with a transformer structure, but that device has a power port with a relatively high inductance and considerable losses. As a result, the power level has to be adjusted to compensate for those losses, and the power port of the sensor has to be considered as part of the matching network between building blocks, which complicates the design. Those concerns are most important for power amplifiers in rf transceivers wherein a power sensor is attached to the output port. Any additional loss associated with the insertion of the power sensor will reduce the power-added-efficiency of the amplifier. This has a direct impact on the power consumption of the transceiver because a major part of the power applies to the power amplifier.
In U.S. Pat. No. 5,041,791 to Ackerman et al., a magnetic resonance RF antenna probe is described that consists of a transmitter coil for transmitting rf energy to a specimen. A receiver coil comprising two anti-phase receiver elements senses the rf energy absorbed or emitted by the specimen. In order to eliminate unwanted coupling between the transmitter and receiver coils, this design calls for rotation of one of the receiver coil elements in order to balance the current induced by the transmitter coil in both receiver elements to zero. Thus, the disclosed device could not be readily fabricated on a substrate using integrated circuit fabrication techniques. U.S. Pat. No. 4,977,366 to Powell and U.S. Pat. No. 4,724,381 to Crimmins disclose sensors for transmission lines that produce a sense signal with a potential that is undesirably dependent on that of the power port. The same restriction applies to the proposal of U.S. Pat. No. 5,420,464 to Krett, where voltage drop across a resistor is used to sense power levels. U.S. Pat. No. 5,461,308 to Jin et al. discloses a power sensing device that takes advantage of the fact that a magnetic field affects the resistance of magneto-resistive material. U.S. Pat. No. 5,057,848 to Rankin et al. discloses orthogonal antenna pairs to sense electromagnetic energy.
The prior art thus lacks important features that are necessary to a signal sensor design with improved efficiency and which can be readily used with monolithic devices fabricated on substrates using integrated circuit techniques. What is needed is a signal sensor having an input port that is configured for minimum loss and impedance, but which provides a sense signal that is only one or two orders of magnitude (i.e., 10-20 dB) less than the input signal, and which has a negligible change in phase. What is especially required is a power sensor that can be fabricated in silicon or the like and used in rf integrated systems.